Devices and processes for heat treating the heat exchanger tubes of nuclear steam generators are known in the the art. Generally, the purpose of these devices and processes is to relieve the tensile stresses that occur in certain portions of these tubes which in turn can result in a phenomenon known as "stress corrosion cracking." However, before the importance of these stress-relieving devices and processes can be fully appreciated, some understanding of the structure and operation of nuclear steam generators is necessary.
Nuclear steam generators are formed from three principal components, including a primary side which circulates water heated from a nuclear reactor, a secondary side, and a tube-sheet which hydraulically separates the primary and secondary sides. The secondary side of the generator contains a plurality of U-shaped heat exchanger tubes and an inlet for admitting a flow of feedwater. The inlet and outlet ends of the U-shaped tubes are mounted in the tubesheet. The primary side includes a divider sheet which hydraulically isolates the inlet ends from the outlet ends of the U-shaped tubes. Hot, radioactive water flowing from the nuclear reactor is admitted into the section of the primary side containing the inlet ends of the U-shaped tubes. This hot, radioactive water flows through the inlets, through the tubesheet, and circulates around the U-shaped tubes which extend within the secondary side of the generator. This water transfers its heat through the walls of the U-shaped tubes to the nonradioactive feedwater flowing through the secondary side of the generator, thereby converting this feedwater to nonradioactive steam which powers the turbines of an electric generator. After the primary side water from the reactor circulates through the U-shaped tubes, it flows back through the tubesheet, through the outlets of the U-shaped tubes, and into the outlet section of the primary side, where it is recirculated back to the nuclear reactor.
Chronic maintenance problems arise in such generators as a result of tensile stresses induced in the walls of the heat exchanger tubes. These tensile stresses may in turn cause the tubes to corrode and to crack as a result of lead to an undesirable "stress corrosion cracking" if the stress is not relieved. The resulting cracks in the tubes can cause the heat exchanger tubes to leak radioactive water from the primary side into the secondary side of the generator, thereby radioactively contaminating the steam produced by the steam generator. Such tensile stresses may occur from either manufacturing or maintenance operations. For example, stress-causing bends are incorporated into the heat exchanger tubes during their manufacture to create their U shape. Stress-causing expansions are routinely induced in various sections of the heat exchanger tubes in the tubesheet and support plate regions, both during the manufacture and maintenance of the generator. Also, stress-causing welds may be placed around the interior walls of the tubes whenever reinforcing sleeves are welded therein. Other tensile stresses occur from the accumulation of sludge deposits in the crevice regions of the generator, such as the annular region between the heat exchanger tubes and the bores in the tube support plates through which the tubes extend. Such deposits can accumulate in these annular regions and expand to such an extent that the tube becomes dented into an ovular cross section in the support plate region. This is known as radial tube denting. A detailed discussion of radial tube denting and maintenance expansions designed to prevent them can be found in U.S. Pat. No. 4,649,492 by Sinha et al., assigned to the Westinghouse Electric Corporation, the entire specification of which is incorporated herein by reference.
In order to prevent corrosion and tube cracking from occurring in the heat exchanger tubes of the generator, both mechanical and thermal stress-relieving processes have been developed. One of the most successful thermal stress-relieving processes is disclosed in U.S. patent application Ser. No. 24,941 filed Mar. 13, 1987 and entitled "Process for Thermally Stress Relieving a Tube" by Bevilacqua et al., assigned to the Westinghouse Electric Corporation, the entire specification of which is incorporated herein by reference. This process provides results in an extremely fast yet reliable process for stress relieving Inconel.RTM. heat exchanger tubes which have had tensile stresses induced therein by bending, denting, tube expansions, or sleeve weldings.
Unfortunately, this particular process is very difficult to implement along tube sections having non-homogeneous thermal conductivity characteristics, such as the sections of the heat exchanger tubes that are surrounded by tube support plates. The sections of the tubes that extend through the tube support plates often need to be stress relieved either as a result of a deliberate tube expansion in this area, or radial tube denting. However, if one attempts to thermally stress-relieve the section of the tube extending through and in contact with such support plates using the '941 method with a stationary heater assembly that is at least as long as the tube section to be heat treated, one of two unsatisfactory results follow. Either the portion of the tube directly in contact with the tube support plate is underheated due to the heat sink properties of the plate, or, if the power of the heater assembly is increased to adequately heat the plate-contacting portion of the tube, the sections of the tube above and below the plate are overheated (i.e., heated to over 1500.degree. F.). Overheating can cause carbides to precipitate in the grain boundaries of the Inconel.RTM. that forms the tube, which renders these portions of the tube brittle and more susceptible to stress-corrosion cracking, thereby defeating the purpose of the thermal stress relief.
U.S. patent application Ser. No. 069,721 filed June 6, 1987 and entitled "Process for Heat Treating a Heat Exchanger Tube Surrounded by a Support Plate" by Cheng and assigned to Westinghouse Electric Corporation (the entire specification of which is incorporated herein by reference), discloses an improved method for thermally stress-relieving a tube which is capable of creating and maintaining a substantially uniform heat gradient across a section of a metallic tube that is characterized by non-homogeneous thermal-loss properties. This improved process is implemented by oscillating a heater assembly in the tube section being treated in accordance with a selected amplitude and period so that the tube section is substantially uniformly heated to the desired heat treatment temperature.
The heater apparatus that implements this process includes a heater probe, a fiberoptic cable connected to a pyrometer, and a probe positioning device. The fiber-optic cable is used to conduct the light of incandescence emanating from the heated tube to the pyrometer. While this heater assembly is capable of heating the tube walls sufficiently to relieve stresses, the inventors have observed that the accuracy of the temperature monitoring instrumentalities in these prior art devices which could bear improvement. Specifically, the applicants have noted that the heat and light emanating from the radiant heating element of the heater probe can sometimes cause the pyrometer to register a temperature reading that is inaccurately high or low, thus creating the possibility that a tube may be underheated or overheated. Another drawback associated with the temperature monitoring instruments used in these probes is that they have only one fiber-optic cable for transmitting the light of incandescence to the pyrometer. Hence these probes can monitor the temperature of the tubes they heat at only one point at one time. Because of the non-uniformities of the heat gradient caused by the heat sink properties of the support plates that surround the tubes, such one-point monitoring can result in an inaccurate temperature reading. To ensure accurate temperature monitoring, the applicants have found that it is important to measure temperature at more than one location in the region of the tube being heated.
Finally, the inventors have noted that problems can arise in the fiber-optic cable that receives the light emanating from the heated tube in these prior art probes. The end of the fiber-optic cable mounted in the heater probe is polished at a 45.degree. angle to receive the light and to reflect it down the axis of the cable to the pyrometer. Where such 45.degree. polishing is used, the fiber-optic cable is mounted deep within the ceramic body of the probe in order to prevent the cable from being over-heated, and a light-collimating port is provided in the ceramic body to conduct light to the 45.degree. polished end. Unfortunately, this arrangement significantly interferes with the conduction of light from the heated tube to the body of the cable. Additionally, this arrangement requires that the fiber-optic cable be assembled in the heater probe as two pieces. One segment is long and runs from the pyrometer to the ceramic body. The other segment, which included the polished 45.degree. surface, is short and extends from the end of the long segment to the port or window in the probe body. The short segment of fiberoptic cable causes significant interference with the conduction of light from the heated tube to the long segment of cable, resulting in numerous additional problems. The outer surface of the fiber-optic cable which faces the tube wall is curved, detracting from optimum light passage. Moreover, it is difficult to position precisely the short segment of fiber-optic cable which tends to rotate and move out of its proper position easily. This reduces the effectiveness of the probe as the 45.degree. surface requires an exact, precise alignment facing the tube wall. The gap created between the two segments of the fiber-optic cables also interferes with the light passage. Finally, the small diameter of fiber-optic cable severely restricts the amount of light conducted through the cable. The effective window area for the cable is only 0.039 inch high by 0.039 inch across a round surface.
To enhance the conduction of light from the heated tube to the end of the cable, the inventors tried to mount the cable within the probe body in a 20.degree. bend to minimize the distance between the polished end of the cable and the wall of the heated tube. However, such bending introduced stresses in the cable which rendered it very fragile. Additionally, polishing the end of 35 the cable at a 20.degree. angle proved difficult and expensive, which increased the cost of properly machining the fiber-optic cable.
Clearly, there is a need for a heater probe that is capable of accurately monitoring the temperature of the tube being heated at a plurality of points by means of a fiber-optic configuration which is both durable and sensitive.